The past three decades have brought increased interest in the use of lasers in material processing applications. Early procedures for material processing and cutting involved optical drilling using continuous wave or relatively long pulse (e.g., 50 to 350 .mu.s) lasers such as C02, ruby and ND:YAG (Neodymium doped Yittrium Aluminum Garnet). These systems, however, required relatively high radiant exposure and resulted in significant alterations to surrounding tissue. As a consequence, lasers could become an effective cutting tool only in areas which did not require high degree of precision or control.
Optical drilling with ER:YAG (Erbium doped YAG) lasers yielded encouraging results in the late 1980s, and has demonstrated its capability to perform as an efficient drill while incurring only relatively low levels of collateral damage to surrounding tissue, provided that no more than one to three pulses per second were applied to the target material. The success of ER:YAG systems, operating in the microsecond pulse duration regime and minimizing thermal damage has also been observed in several areas of applications in material processing and medicine, and can be attributed to the high absorption coefficient of these materials at the particular wavelengths characteristic of the Er:YAG system (2900 nm), when used in combination with the relatively short pulse durations and at low pulse repetition rates.
Laser systems adapted to hard tissue processing, such as dentin and enamel removal in dental applications are disclosed in: 1. Hibst R, Kelly U. Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement of the Ablation Rate. Laser Surgery and Medicine 1989, 9:352-7; and, 2. Keller U, Hibst R. Experimental studies of the application of the Er:YAG laser on dental hard substances: II. Light microscopy and SEM investigations. Lasers in Surgery and Medicine 1989; 9:345-351.)
Both pulsed CO2 and Er:YAG are disclosed in: Walsh, J. T., Flotte, T. J., Anderson, R. R., Deutsch, T. F., "Pulsed CO.sub.2 Laser Tissue Ablation: Effect of Tissue Type and Pulse Duration on Thermal Damage, "Lasers in Surgery and Medicine, Vol. 8, pp.108-118, 1988; Walsh, J. T., Flotte, T. J., Deutsch, T. F., "Er:YAG Laser Ablation of Tissue: Effect of Pulse Duration and Tissue Type on Thermal Damage, "Lasers in Surgery and Medicine, Vol. 9, No. 4, pp. 314, 1989; and Walsh, J. T., Deutsch, T. F., "Er:YAG Laser Ablation of Tissue: Measurement of Ablation Rates, "Lasers in Surgery and Medicine, Vol. 9 No. 4, pp. 327, 1989.
A Ho:YSGG laser system is disclosed in Joseph Neev, Kevin Pham, Jon P. Lee, Joel M. White, "Dentin Ablation with Three Infrared Lasers," Lasers in Surgery and Medicine, 18:121-128 (1996).
The laser systems disclosed (Er:YSGG, HO:YSGG, and Pulsed CO2) all operate in the IR region of the electromagnetic spectrum and are pulsed in two different regimes: about 250 microsecond pulse durations for the ER:YSGG and HO:YSGG lasers, and about 150 microsecond pulse durations for the CO2 system.
While the disclosed removal rate is in the range of approximately tens of micrometers per pulse, the disclosed laser systems exhibit wavelength dependent absorption and result in high removal rates by operating at pulse energies in excess of 30 millijoules per pulse and often on the order of a few hundreds of .mu.J per pulse. Enhancing material removal by increasing laser power is, however, accompanied by increased photothermal and photomechanical effects which causes collateral damage in adjacent material. In addition, increasing power leads to plasma de coupling of the beam, e.g., incident laser energy is wasted in heating the ambient in front of the target. High intensity pulses additionally cause very loud acoustic snaps, when the laser pulse interacts with tissue. These snaps or pops include a large high frequency component which is very objectionable to a user or, in the case of a medical application, to a patient. In addition to the psychological impact of such noise, these high frequency snaps are able to cause hearing loss in clinicians when repeated over a period of time.
U.S. Pat. No. 5,342,198, to Vassiliadis, et al. discloses an ER:YAG IR laser system suitable for the removal of dentin in dental applications. The laser produces a pulsed output having a beam with a pulse duration in the range of several tens of picoseconds to about several milliseconds. Although disclosed as being efficient in the removal of dentin and dental enamel, the mechanism by which material removal is effected is not understood. Significantly, however, the only laser systems disclosed as suitable for the process are those which operate at wavelengths (1.5 to 3.5 microns) that have proven to be generally effective for enamel interaction. Thus, the absorption characteristics of the material target are of primary concern to the removal rate. In addition, high energy levels are required to remove enamel and dentin, leading to the problem of thermal damage and acoustic noise.
Additional possibilities for the application of lasers to the field of dentistry in particular, and to hard tissue ablation in general, have been proposed by the use of excimer lasers that emit high intensity pulses of ultraviolet (UV) light.
Several such pulsed UV excimer laser systems, typically with pulse durations in the approximately 1 to 125 nanosecond range are disclosed in:
1. Neev J, Stabholz A., Liaw L. L, Torabinejad M, Fujishige J. T., Ho P. H, Berns M. W., "Scanning Electron Microscopy and Thermal characteristics of Dentin ablated by a short-pulse XeCl Laser", Lasers in Surgery and Medicine; PA1 2. Neev J, Liaw L, Raney D, Fujishige J, Ho P, Berns M. Selectivity and efficiency in the ablation of hard Dental tissue with ArF pulsed excimer lasers. Lasers Surgery and Medicine 1991; 11:499-510; PA1 3. Neev J, Raney D, Whalen W, Fujishige J, Ho P, McGrann J, Berns M. Ablation of hard dental tissue with 193 nm pulsed laser radiation: A photophysical study. Spie proceedings, January 1991; and PA1 4. Neev J, Raney D, Whalen W, Fujishige J, Ho P, McGrann J, Berns M. Dentin ablation with two excimer lasers: A comparative study of physical characteristics. Lasers Life Sci 1992; 4(3):1-25. Both the short wavelengths and nanosecond range pulse durations used by excimer lasers contribute to define a different regime of laser-tissue-interaction. Short wavelength ultraviolet photons are energetic enough to directly break chemical bonds in organic molecules. As a consequence, UV excimer lasers can often vaporize a material target with minimal thermal energy transfer to adjacent tissue. The resultant gas (the vaporization product) is ejected away from the target surface, leaving the target relatively free from melt, recast, or other evidence of thermal damage.
Another important characteristic of UV excimer lasers is that materials which are transparent to light in the visible or near infra-red portions of the electromagnetic spectrum often begin to exhibit strong absorption in the UV region of the spectrum. It is well established that the stronger a materials absorption at a particular wavelength, the shallower the penetration achieved by a laser pulse having that wavelength. Thus, in many types of materials, a pulse typically only penetrates to a depth in the range of from about 10 to about 100 micrometers. By simply counting pulses, great precision can be achieved in defining removal depths. In addition, organic tissue is strongly absorbent in the UV wavelengths (193 nm for ArF, for example) therefore allowing the laser-tissue interaction region to be controlled with great precision.
Notwithstanding the relatively damage free material removal characteristics of UV excimer lasers, these systems suffer from several disadvantages which limit their applicability to biological tissue processing. The reports of damage free tissue removal result from evaluations performed on single pulses, or on pulses with a very low repetition rate (typically about 1 to 10 Hertz). Because of the low volumetric removal per pulse of excimer systems (material removed per unit time is poor), efficient material removal can only be accomplished by high pulse repetition rates. However, when the pulse repetition rate exceeds about 3 to 5 Hertz, considerable thermal and mechanical collateral damage is observed. While UV photons are sufficiently energetic to directly break chemical bonds, they are also sufficiently energetic to promote mutagenic effects in tissue irradiated at UV wavelengths, raising concerns about the long term safety and health of a system operator. The scattered light produced by excimer lasers also presents a significant threat to the clinician and/or the patient. Even low intensity scattered radiation, with wavelengths below 300 nanometers, is able to interact with the ambient environment to produce atomic oxygen and other free radicals. These can, in turn, react with the lens and cornea of the eye, producing cataracts, and produce burns on the skin equivalent to sun burns. As a consequence, excimer laser systems have been found to be most suitable for inorganic material processing applications, such as thin coating patterning or dielectric or semiconductor material etching.
In addition, the operational parameters of excimer laser systems are such that material removal remains a wavelength and beam energy dependent process (although weakly dependent on wavelength). Even when pulsed in the tens of nanoseconds pulse duration regime, excimer lasers are configured to deliver energy in the range of from about 10 to about 1000 millijoules per pulse. At the higher energies, excimer lasers suffer from the same problems caused by plasma decoupling and pulse to pulse interaction as IR lasers. Additionally, as pulse energy increases, so too does the intensity of the associated acoustic snap.
Neev et al. (University of California Case No. 95-313-1) U.S. patent application Ser. No. 08/584,522 described a Selective material removal processing Ultra Short Pulse Lasers (USPL) system in combination with a feedback system and with higher pulse repetition rates. This invention is directed to a system for efficient biological tissue removal using ultra short pulses. Such pulse durations are shorter than the characteristics electron-phonon energy transfer time, thus minimizing collateral thermal damage. The method also requires that plasma is formed and decayed so that a thin layer portion of the material is removed. The plasma formation step is then repeated at a pulse repetition rate greater than 10 pulses per second until a sufficient depth of material has been removed with little transfer of thermal or mechanical energy into the remaining material due to the shortness of the pulse duration. The preferred wavelength for that invention is in the range of 200-2500 nm. The laser specified in that patent application is a Chirped Pulse Amplifier (CPA) Solid-state laser.
That patent further specified that the laser system is comprised of a feedback means for analyzing material characteristics in response to interaction between the laser pulses. The envisioned feedback means comprises a spectrograph to evaluate the plasma formed by each pulse. The feedback means is operatively coupled to the laser. The laser operatively responds to the control signal such that the laser ceases operation upon receipt of the control signal. The feedback means also comprises an optical tomograph which optically evaluates the amount of target material removed by each pulse.
This invention should work well in many applications. Unfortunately, the equipment for the ultrashort pulse duration is very expensive (currently, over $100,000 and often two or three times that amount) and still requires many components and careful maintenance. The systems are also very large and delicate and require large volume for storage and expert maintenance at this stage of the technology. Also the interaction is not very selective nor highly sensitive to the targeted material type but rather ablate most materials. This, in turn, effects some risk of over ablating or removal of unintended structures. The highly interactive nature of the ultrashort pulse process possess additional problems to attempts to deliver the ultrashort pulse beam to the target. Most optical fibers as well as mirror and lenses could easily be damaged if ablation threshold is exceeded (either through narrowing of the beam spot size, an increase in pulse energy, or compression of the pulse duration). Thus ultrashort pulses are hard to deliver through most conventional delivery systems.
An additional problem is that ultrashort pulse lasers are currently achieved principally in the near IR region of the electromagnetic spectrum. This is a highly transparent region for most biotissue material. Consequently, some portion of the radiation propagates linearly into the material and is not confined to the surface. This additional energy propagating into the target may then encounter more absorbing structures (for example the blood vessels in the retina) and will then result in a secondary--unintended--ablative interaction, posing risk to the patients or to the material being processed.
U.S. Pat. No. 4,907,586 issued to Bille and Brown for "METHOD FOR RESHAPING THE EYE", disclosed a method for modifying tissue with a quasi-continuous laser beam to change the optical properties of the eye which comprises controllably setting the volumetric power density of the beam and selecting a desired wavelength for the beam. Tissue modification is accomplished by focusing the beam at a preselected start point in the tissue and moving the beam's focal point in a predetermined manner relative to the start point throughout a specified volume of the tissue or along a specified path in the tissue.
More particularly, the method describes a sequence of uninterrupted emissions of at least one thousand pulses lasting for at least one second. The pulses were specified lasting approximately one picosecond (1 ps) in duration and of less than 30 micro joules (30 .mu.J).
The invention disclosed in U.S. Pat. No. 4,907,586 should work well for reshaping the eye, but is confined to the region of 1 ps and thus also involves the generation of ultrashort pulses and their relative low thermal and mechanical deposition of energy during the single pulse interaction. This device thus requires the use of expensive ultrashort pulses with all the specified limitations mentioned above. In addition, this invention is limited to relatively low energies of 30 .mu.J, which require a very tightly focused beam to affect tissue ablation. The invention will thus not work well for larger areas or for high volume removal rates, which are required in many applications, e.g., dentistry, surgery, etc. This invention is also limited with regards to its ability to deliver pulses through optical fibers, hollow waveguides or conventional optics since the very shorted pulses of 1 ps are also very reactive and will interact with most material used as deliver media. Consequently, specialty optics has to be used and conventional lenses and mirrors as well as optical fiber and conventional hollow waveguides cannot be used.
In the present invention, the inventor has recognized that a much wider range of pulse durations of up to approximately several hundred microseconds will allow the thermal diffusion to remain confined to within a distance of only a few micrometer of the ablated crater. Thus, the present invention is concerned with pulses up to several milliseconds long. With a combination of short pulse to pulse separation and with new requirement on both the number and the rate of the incident sequential pulses, the present invention allows large volume removal or volume processing with substantially little damage to surrounding regions of the target.
The present invention thus allows the use of pulse laser systems that are substantially less expensive and in many instances safer and more efficient than those described by other inventions, while achieving unprecedented volume removal rate, high precision, high efficiency and minimal thermal or mechanical collateral damage.